This invention relates to monopole/slot antennas, and more particularly to such antennas where the height of the monopole is reduced by the addition of compensating features and the radiation pattern "steering" capability is increased with modification to the slot and stripline configurations.
By way of background, unidirectional antennas with pattern rotation or "steering" capability are known in the art. One such antenna is described in a thesis entitled "The Hybrid Slot Antenna" by Thomas Allen Cwik, B.S., University of Illinois, 1979. In that publication, beginning at page 66, there is described a four port hybrid slot antenna having upper and lower ground planes and a stripline midway therebetween. The upper ground plane has an annular slot surrounding a conductive disk located centrally of that plane. Orthogonal rectangular slots intersect the annular slot at their centers. The stripline consists of a pair of stripline portions parallel to the ground planes and midway therebetween. The stripline portions are oriented 90.degree. to each other and 45.degree. to the longitudinal axis of each rectangular slot and crossing directly under the center of the disk. A paper thin piece of mylar is placed between the stripline portions to achieve isolation. The stripline is fed through ports at the ends of the stripline portions. The antenna had no monopole. According to the publication, by successively feeding two adjacent ports in phase and with equal magnitudes, the antenna produces a cardioid pattern successively steered 90.degree..
The antenna of the present invention has certain advantages over that described in the publication, in that it provides beam steering with improved structure and performance attained by incorporating important features described in said co-pending application, such as an offset stripline and capacitively loaded slot.
Hence, in said co-pending application Ser. No. 569,265 there is described a reduced height monopole/slot antenna with substantial advantages over other monopole/slot antennas. In accordance with the antenna of that application, the height of the monopole is substantially reduced while maintaining the impedance and cardioid radiation characteristics over a wide bandwidth. This is accomplished by offsetting the stripline toward the slotted ground plane, and capacitively loading the slot. The result is a wide bandwidth antenna that develops a highly directive cardioid radiation pattern. It has low volume, excellent form factor, and easy producibility. The wide bandwidth occurs at a low input VSWR over an extremely wide frequency range, and a cardioid pattern in the antenna ground plane with a deep null is maintained over a wide bandwidth. The monopole element, which protrudes above the antenna in the full height monopole/slot antenna, is significantly reduced in height by electrical loading. Therefore, for many applications, the antenna of said application can be flush mounted with no protrusions above the mounting surface.
The antenna of the present invention incorporates these important features of the antenna of the co-pending application, including the offset stripline and capacitively loaded slot, while providing the additional advantage of more beam steering flexibility.
In accordance with the present invention, the slot is formed of two orthogonal radiating rectangular slot portions in the upper ground plane. The stripline is formed of two stripline portions, one orthogonal to the other with the stripline oriented at 45.degree. out of alignment with the longitudinal axes of the slot portions. The monopole extends from the intersection of the stripline portions and through the center of intersection of the cavity portions orthogonal to the upper ground plane. Connectors or ports are provided at each end of each stripline portion for selectively feeding the stripline. As with the invention of the co-pending application, the monopole is top hat loaded, the stripline is offset toward the upper ground plane, and the slot is capacitively loaded. Through various combinations of feeding the stripline, various beam positions and radiation patterns are obtained, all while retaining the advantages described in the co-pending application. The result is a reduced height monopole/crossed slot antenna with stable, deep null, cardioid radiation patterns, with beam steering capability, and a low VSWR over a wide bandwidth. The antenna has low volume, excellent form factor, and easy producibility.